Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing, 100083, China.
Sinopec Petroleum Exploration and Production Research Institute, 100083, China.
J Nanosci Nanotechnol. 2021 Jan 1;21(1):139-155. doi: 10.1166/jnn.2021.18462.
Fractal dimension is closely related to the nanoscale pore structure of shale, and it also has an important influence on the gas content of shale. To investigate the correlation between the fractal dimension and the methane (CH₄) bearing features of shale, seven Permian shale samples were analyzed with field emission scanning electron microscopy (FE-SEM), low temperature nitrogen (N₂), carbon dioxide (CO₂) and CH₄ adsorption and on-site gas desorption experiments. Based on the N₂ adsorption and desorption data, we proposed a new method to better determine the gas adsorption stage at different relative pressure (/) points in the multilayer adsorption or capillary condensation stage. On this basis, two fractal dimensions, (representing the surface roughness) and ₂ (representing pore irregularity), were obtained. By correlating the fractal dimensions and nanoscale pore structure parameters, we found that does not correlate with the pore structure parameters except for the micropore volume. Influenced by the aggregation of porous and nonporous materials, ₂ has a positive linear relationship with the specific surface area (SSA) and micropore volume but has a negative linear correlation with the average diameter of pores. is negatively correlated with water saturation and positively correlated with free CH₄ content. The CH₄ adsorption content is positively correlated with ₂. By fitting the on-site desorption data, the positive correlation between the total desorbed CH₄ content and the desorbed CH₄ content in stage 2 and ₂ was also confirmed. ₂ better reflects the CH₄ adsorption capacity of organic-rich shale than . However, can be used to reflect the influence of shale surface properties on water saturation and to indirectly reflect the free CH₄ content in shale. The fractal dimension ( and ₂) is a clear indicator of the total free and adsorbed CH₄ content, but cannot indicate the desorbed CH₄ content at different stages.
分形维数与页岩的纳米级孔隙结构密切相关，对页岩的含气量也有重要影响。为了研究分形维数与页岩甲烷（CH₄）赋存特征的相关性，采用场发射扫描电子显微镜（FE-SEM）、低温氮（N₂）、二氧化碳（CO₂）和 CH₄吸附及现场解吸实验对 7 个二叠纪页岩样品进行了分析。基于 N₂吸附和解吸数据，提出了一种更好地确定不同相对压力（/）点下多层吸附或毛细凝聚阶段中气体吸附阶段的新方法。在此基础上，得到了两个分形维数， （代表表面粗糙度）和 ₂（代表孔隙不规则度）。通过关联分形维数和纳米级孔隙结构参数，发现 除了微孔体积外，与孔隙结构参数无关。受多孔和非孔材料聚集的影响， ₂与比表面积（SSA）和微孔体积呈正线性关系，与孔径平均值呈负线性关系。 与水饱和度呈负相关，与游离 CH₄含量呈正相关。CH₄吸附量与 ₂呈正相关。通过拟合现场解吸数据，也证实了总解吸 CH₄含量与第 2 阶段和 ₂的解吸 CH₄含量之间的正相关性。 ₂比 更能反映富有机质页岩的 CH₄吸附能力。然而， 可用于反映页岩表面性质对水饱和度的影响，间接反映页岩中游离 CH₄的含量。分形维数（ 和 ₂）是总游离和吸附 CH₄含量的明确指标，但不能指示不同阶段的解吸 CH₄含量。
